Last week, I noticed two stains on our living room ceiling beneath our master bathroom: one about 18"x3" ran along the joint of the sheetrock and cracks were beginning to show along the tape. The other was about 4"x1." Plumbers came out and determined the larger stain was under a double vanity and the smaller was closer to the toilet. After searching for a source in the bathroom, they cut into the ceiling and still couldn't find any active leak. They suggested it could be from the roof since we'd had recently had a lot of rain. (NOTE: it has not rained again since the first stain appeared.)

I called a contractor who came out, looked at the stains, went into the attic and tried to find a source using a moisture meter: nothing. He said it couldn't be the roof since the attic and upstairs ceilings were fine. After he left, I noticed another stain had developed in the living room: a perfect concentric circle about 4 inches in diameter. He has since cut a bigger hole in the ceiling and we still see no source of water. None of the wood appears stained or moldy, and all the pipes stay dry when we run water from anything in the master bath. Also, we saw that the circular stain is directly under a nail in a support beam that has no plumbing above. It's as though the nail dripped and the circle bled out from it.

Finally, while he was here yesterday, another stain appeared.....but the pipes and area above the ceiling are still dry.

The house is only 3 years old and extremely well-built. We've never had any problems before. Now we're about to sell it, and this happens! HELP!

Mystery stains can be a real headache to find. If the house is only 3 years old and really well built as you say then I'll just describe the most likely culprits.

First - check the toilet in the bathroom above. Does it rock, wiggle or move? If so then the bolts that attach it to the flange have worked loose and the wax ring that seals it may be leaking. If it dosn't wiggle then the seal is probably ok.

Most likely the leaks are with caulk around your shower or tub. This is most common. Running water to test doesn't help diagnose the problem. The problem could occur only when weight is in the shower or tub or when you shower you splash the water differently than when you just run it for a test.

Its not likely in any plumbing supply lines or drain line inside the walls/ceiling.

So, check every caulk joint. It could even be a pinhole. Check any caulking outside the shower, where the floor meets the shower curb/pan or the edge of the tub. I would go ahead and re-caulk everything. I would bet this is the issue.

I'd maybe put some food in a bowl up in your ceiling somewhere and see if it disappears. Maybe a stray cat or something is making a home up there. Maybe don't put any water up there with the food or you might see a lot more stains.

Welcome Virginia:
You are running cold water through the ceiling space which is warm; this causes condensation which drips down and gives the appearance of a leak. If you could insulate the pipes so the air couldn't get to them, you would probably solve the problem.
Cover the water lines with closed cell foam insulation and drain pipes with fiberglass batts wrapped around them and secured.
The secret is to insulate or ventilate, so you don't condensate. In your case the ventilation doesn't seem to be possible.
Glenn

Virginia: Did you ever find out the source of the leak? I am having this very same issue in my house as we speak. My place is only 2.5 years old and water just started dripping through our kitchen ceiling (master bath is directly above the kitchen). The weird thing is, I was showering the other night and a decent amount of water was dripping through the drywall joint in the ceiling in two spots. The next morning my wife was giving my son a bath and no water leaked. Later that day my wife showered and in the evening I showered and still no water leaked. This is a mystery. Thanks.